1. Field of the Invention
The present invention relates to a plated product having a copper thin film formed thereon by electroless plating, and more particularly to a plated product having a copper thin film formed thereon by electroless plating that serves as a seed layer for forming ULSI ultrafine copper wiring (damascene copper wiring).
2. Description of the Related Art
As a method for deposition of a copper film for ULSI ultrafine copper wiring (damascene copper wiring), electroless copper plating is expected to replace the current sputtering and electrolytic copper plating methods.
When electroless copper plating was performed on a semiconductor wafer or other mirrored surface in the past, it was difficult to acquire sufficient adherence of the deposited plating film. In addition, the plating had low reactivity, and uniform plating over the entire surface of the substrate was difficult to obtain. Conventionally, for example, when a copper seed layer was formed by electroless plating on a layer of a barrier metal such as tantalum nitride, there was a problem because it was difficult to form the plating layer uniformly, and the adherence thereof was insufficient. Moreover, also when forming a copper seed layer by electroless plating on a barrier metal layer of an elemental metal such as tungsten or molybdenum, there was still a problem because it was difficult to form the plating layer uniformly, and the adherence thereof was insufficient. Therefore, a catalyst-imparting step prior to electroless plating was required in order to form a uniform copper seed layer on a barrier layer of tantalum nitride and the like.
The present inventors had already discovered that by adding a water-soluble, nitrogen-containing polymer with a low weight-average molecular weight (Mw) as an additive to the electroless copper plating solution, and by attaching a catalyst metal to the substrate of the object to be plated or by forming a catalyst metal as a film on the outermost surface of the object to be plated before immersion in the plating solution, and then immersing the object to be plated in the plating solution to adsorb the polymer into the catalyst metal via the nitrogen atoms, the plating deposition rate was controlled, and the crystals were dramatically micronized, thus enabling the formation of a uniform thin film with a thickness of 15 nm or less on a mirrored surface such as that of a wafer (Japanese Patent Publication No. 2008-223100). Moreover, in the examples of that invention the inventors demonstrated that by first forming a film of catalyst metal on the outermost surface of the object to be plated and then immersing the object to be plated in the plating solution to adsorb the polymer into the catalyst metal via the nitrogen atoms, the plating deposition rate was controlled, and the crystals were dramatically micronized, thus enabling the formation of a uniform thin film with a thickness of 6 nm or less on a mirrored surface such as that of a wafer.
In such a method, i.e., a method of providing a copper seed layer for the formation of damascene copper wiring by electroless plating after forming a catalyst metal film, a barrier layer to prevent copper diffusion must be formed on the substrate prior to and separately from the catalyst layer. This means that a bilayer consisting of the barrier layer and the catalyst metal layer must be formed prior to the formation of the copper seed layer. Thus, it became clear that application of this method to the actual step of fabricating ultrafine wiring wherein the film thickness cannot be increased will be very difficult.